KLRK1 and neuroblastoma: Cell surface expression of HLA-I and NKG2D ligands and NKG2D ligands/HLA-I ratio on medulloblastoma cell lines (HTB-185, HTB-186, HTB-187), neuroblastoma cell line (NB1691) and erythroleukaemia cell line (K562) measured by mean fluorescence intensity (MFI).